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referred to and expressed it correctly was a German physician, J. E. Mayer, of Heilbronn, in the year 1842." Then he says, "I myself, without being acquainted with Mayer or Colding, and having first made the acquaintance of Joule's experiments at the end of my investigations, followed the same path."[1]

We have now briefly sketched the birth of the principle of the conservation of energy in the minds of Mayer, Joule and Helmholtz. After examining the facts we are convinced that these great physicists were independent discoverers. Lack of time prevents us from making reference to forerunners like Count Rumford,[2] Sadi Carnot,[3] Seguin,[4] Mohr[5] and to the Dane, named Colding,[6] who in 1843 gave utterance to the law before the Academy in Copenhagen. We pass by the researches of Rankine, to whom we owe the expression, "conservation of energy," as well as William Thomson's doctrine of the "dissipation of energy."

We come now to the second part of this paper—the application of the conservation of energy and other principles of physics to the examination of the age of the sun and of the earth. The two problems are closely interrelated; the earth-age is measurable by the sun-age.

Before the time of the Scotch geologist, James Hutton, some 6,000 years was believed to indicate the age of the earth, and, indeed, of the entire universe. The advent of the uniformitarian school of geologists marks a radical departure from the old estimates. The pendulum swings from one extreme to the other. Boundless distances of time were now drawn upon. So great an antiquity of the earth seemed to reveal itself to geologists, as to defy all attempts at measurement. In the further pursuit of Hutton's line of investigation, Play fair and Lyell were unable to discover among the records of the earth and in planetary motion either a beginning or an end of the present order of things. They found no indication of infancy or decaying old age.[7]

This convenient doctrine of infinite durability came to be rudely attacked by the physicists. Here, as in the history of the conservation of energy, the earliest investigator is Robert Mayer. To be sure, he

  1. "Popular Lectures," by H. Helmholtz (transl. by E. Atkinson), New York, 1897, p. 167.
  2. "The Complete Works of Count Rumford," published by the American Academy of Arts and Sciences, Boston, Vol. I., pp. 481-488.
  3. "Réflexions sur la puissance motrice du feu," 1824, reprinted in Ostwalds Klassiker, No. 37; English translation by R. H. Thurston appeared in 1890.
  4. "De L'influence des Chemins de Fer," Paris, 1839, pp. 378-397.
  5. "Allgem. Theorie der Bewegung," Braunschweig, 1869, pp. 80-84.
  6. A. Colding, Det. Kongel. dansk vidensk. selsk. naturv. ogmath. afh. (5), II., 1843, p. 121, 167.
  7. Sir Archibald Geikie, presidential address before British Association, in Report, British Association for the Advancement of Science, 1892, Vol. 62, pp. 3-26; Smithsonian Report, 1892, p. 124.